Pb with responsive image sizes in WP 4.4

I handle the context like this:

function wpse_216001_srcset() {
    // generate your Srcset here and return 
}
function wpse_216001_sizes() {
    // generate your sizes attribute here and return
}

add_filter( 'wp_calculate_image_sizes', 'wpse_216001_sizes', 10 , 2 );
add_filter( 'wp_calculate_image_srcset', 'wpse_216001_srcset', 10 , 5);

// call one of the standard WP image output functions here

remove_filter( 'wp_calculate_image_sizes', 'wpse_216001_sizes', 10 );
remove_filter( 'wp_calculate_image_srcset', 'wpse_216001_srcset', 10 );

You can either do this in your template or make a wrapper function in your functions.php and then call this as a template tag.